1. Field of the Invention
This invention relates to an optoelectronic coupler. Two applications for the coupler are as an optical modulator or as an optical detector.
Recent advances in technology have resulted in an emphasis on the photonics field--the field which concerns the generation, manipulation, modulation and detection of light and optical waves, where light and optical are taken to refer to the ultraviolet, visible, near-infrared and mid-infrared regions (up to wavelengths of approximately 15 .mu.m) of the electromagnetic spectrum. There has also been a corresponding increase in interest in photonic devices.
Specifically, optical modulators which modulate the properties of optical waves incident upon them and detectors which sense the strength of incident waves are two fundamental building blocks for almost all photonics systems. For example, optical fibers are becoming more prevalent with each passing day, and, in such a system, information is communicated by encoding the information on a light beam, transmitting the beam to the receiver, and decoding the received beam. The encoding can be achieved by using a constant light source modulated by an optical modulator, and a detector is required to decode the received beam. As another example, as computers become more powerful, they are becoming communications-limited rather than processor-limited. That is, the bottleneck on computer speed tends to lie more and more in the time required to communicate between various parts of the computer rather than in the time required to perform the computations. One potential solution is to use optical rather than electronic interconnects since photonics technology offers a potentially large speed advantage. In such an approach, optical modulators and detectors may be used to encode and decode bits in much the same manner as in fiber systems. As a final example, the advent of mass storage devices and high bandwidth communications channels is allowing our society to move towards picture-based communications, the explosion in the number of fax machines and the amount of television programming being prime examples, and the display and capture of these images requires both modulators and detectors. In a fax machine, detectors may be used to capture the image on the transmit side while modulators may be used to recreate the image, a la laser printers, on the receive side. Detectors and modulators may play similar roles in the transmission of video. As a direct result of these technological advances, there is an ever increasing demand for these devices and improvements in these devices.
However, devices such as detectors and modulators which bridge the gap between electrons and photons must rely on fundamental physical phenomena. In the context of this invention, the phenomena relied upon are primarily the coupling of optical waves to one another, the application or sensing of electronic effects through electrodes, and the interaction of photons and electrons in special material systems, such as semiconductors or electro-optic materials. Therefore, advances in these devices depend in large part on either designs which enhance the basic physical effects or practical advances, such as improvements in device cost, reliability, fabrication, ease of operation, etc.
2. Description of the Related Art
Since the invention lies at the intersection of several fields, the related art may also be divided into distinct areas. For convenience, complete reference citations are collected in the last section of the description of the preferred embodiment.
One area of related art concerns the design of gratings to couple between optical waves. For example, [Maystre, et. al., 1978], [Magnusson and Wang, 1993], [Delort and Maystre, 1993] and [Vincent, 1993], the teachings of which are incorporated herein by reference, describe approaches by which the detailed optical properties of the structures described herein may be calculated. These descriptions are complete electromagnetic treatments and include the excitation of surface plasmons, total-internal-reflection (TIR) guided waves, and surface evanescent waves. For convenience, the term local wave will be used to refer to these three types of waves. [Campbell, 1993] theoretically describes the enhancement of light absorption in textured surfaces using a geometrical approach. [Sambles, et. al., 1991] describes the general theory of optical excitation of surface plasmons, including the use of periodic structures, and [Bryan-Brown, et. al., 1991] describes the coupling of surface plasmons to each other. While the teachings of these references may be used to design certain aspects of the current invention, the references themselves are primarily directed towards the purely optical coupling of waves via static structures. The issue of dynamic operation of the devices is unsatisfactorily addressed, as are any electrical aspects.
There are devices which rely on both the coupling of optical waves and some sort of electrical functionality. One application area is that of waveguide modulators. [Simon and Lee, 1988] and [Caldwell and Yearman, 1991] describe dynamic coupling between a wave external to the device and a TIR guided wave or a surface plasmon, respectively. However, the coupling is achieved by prism-coupling or frustrated total internal reflection coupling, which has several practical disadvantages compared to the approach of grating coupling used in the current invention. In the area of grating modulators, [Evans and Hall, 1990] and [Collins, et. al., 1990] both teach the use of a grating to couple between optical waves and, furthermore, the coupling efficiency is modulated by varying a voltage impressed across part of the structure. However, the entire grating is held at one potential and the voltage difference is impressed between the grating and another part of the structure, typically the bulk of a substrate. This is unacceptable because forming electrodes in this fashion and then impressing a voltage across the bulk of the device results in a slow operating speed for the device. A similar situation exists with respect to [Magnusson and Wang, 1993], [Wang and Magnusson, 1993], and [Rosenblatt, 1992]. They teach the use of a grating as a coupling device and suggest methods for electrically varying the optical properties of the grating. However, the electrodes are not adapted for fast operation of the device, as discussed previously. Furthermore, an additional structure is often required to achieve the electrical function, resulting in a more complicated device. In the area of detectors, [Brueck, et. al, 1985] has investigated the use of gratings to couple the incident light to the detector active region, thus increasing the quantum efficiency of the detector. However, as in the devices discussed above, the electrode structure used to sense the generated photocurrent is not adapted to permit high speed operation of the device.
Another area of related art is the general field of optical modulators. As a representative sample of the general literature, [Lentine, et. al., 1989], [Pezeshki, et. al., 1990], [Treyz, et. al., 1990], and [Xiao, et. al, 1991] all teach types of optical modulators which are not directly related to the current invention. In particular, they differ from the current invention in at least one of the following aspects. First, some of the devices are unsatisfactorily slow due to the electrode structure used. Second, some are not based on the coupling of optical waves. Third, none of the devices combine the fast electrode structure and the optical coupling device into a single structure. Finally, many of the devices are not based on VLSI fabrication techniques and so cannot take advantage of the existing manufacturing base and also cannot be as easily integrated with other VLSI circuits.
A final area of related art is the use of specially adapted electrodes. In the modulator area, interdigitated electrodes have been used to apply voltage patterns across electro-optic materials. [Alferness, 1982] and [Hammer, et. al., 1973] teach the use of such electrodes to modulate the optical properties of a waveguide, with the resulting variations in the waveguide controlling the coupling of one waveguide mode to another. However, the electrodes in these cases couple the two modes only indirectly and the restriction that both modes be internal to the waveguide makes this approach unsuitable for the applications of the current invention. Specialized electrode structures have also been used in detectors, particularly metal-semiconductor-metal (MSM) detectors, to increase the speed of these devices. For example, see [Alexandrou, et. al., 1993], [Bassous, et. al, 1991], [Chou and Liu, 1992], [Klingenstein, et. al., 1992], and [Soole and Schumacher, 1991]. [Ghioni, et. al., 1994] have also used specialized electrodes in a nano-detection structure based on a lateral series of PIN diodes. However, in none of these devices are the electrodes adapted to couple the incident optical wave to a local wave in the detector active area, which would result in a significant increase in efficiency.